This invention relates to windows, and in particular to conformal windows used in aircraft sensor systems.
An optical sensor receives radiated energy from a scene and converts it to an electrical signal. The electrical signal is provided to a display or further processed for pattern recognition or the like. Optical sensors are available in a variety of types and for wavelengths ranging from the ultraviolet, through the visible, and into the infrared. Optical sensors are used in a variety of commercial and military applications. In some applications the optical sensors are fixed in orientation, and in others the optical sensor is movable such as by a pivoting motion to allow sensing over a wide angular range.
The optical sensors generally employ a photosensitive material that faces the scene and produces an electrical output responsive to the incident energy. The photosensitive material and remainder of the sensor structure are rather fragile, and are easily damaged by dirt, erosion, chemicals, or high wind velocity. The sensor is placed behind a window through which it views the scene and which protects the sensor from such external effects. The window must be transparent to the radiation of the operating wavelength of the sensor and resist attack from the external forces. The window must also permit the sensor to view the scene over the specified field of regard. This field of regard is the angular extent over which the sensor must be able to be pointed to view the scene. The field of regard may extend over wide angles and in two rotational directions. For example, a look-down sensor on a high-speed aircraft must have a field of regard that extends over large specified angles from front-to-back (elevational angle) and from side-to-side (azimuthal angle).
The window would ideally introduce minimal wavefront distortion of the scene over the field of regard of the sensor, particularly if the sensor is an imaging sensor. The larger and thicker the window, the more likely is the introduction of significant wavefront distortion. Where there is wavefront distortion, as is always the case to some degree, it is desirable that such wavefront distortion be of a predictable, regular type that may be compensated for with an optical device placed between the window and the sensor or by electrical circuitry or processing.
A wide variety of sensor windows have been used in various aircraft applications. In many cases such as low-speed helicopters, flat windows are acceptable. Windows that are shaped as segments of spheres or conic sections are used in aircraft applications, but for these windows the wavefront distortion tends to be high if the pupil of the sensor is large or not at a specific location, or if the field of regard is large. In all of these window types, if the window must be wide in order or must project a substantial distance into an airflow to permit a large field of regard, the aerodynamic drag introduced by the window is large.
For applications involving aircraft operating at high speeds, the window should be relatively aerodynamic such that the presence of the window extending into the airstream does not introduce unacceptably high and/or asymmetric aerodynamic drag to the vehicle. A conformal window is therefore beneficial to reducing drag and increasing the range of the aircraft. Existing conformal windows introduce large wavefront distortions into the sensor beam, particularly for high azimuthal pointing angles of the sensor.
There is a need for an improved window to be used in demanding applications such as look-down and side-looking sensor windows in high-speed aircraft. The present invention fulfills this need, and further provides related advantages.